Fix gschema picture-options value 'zoom'
As documented on
- /usr/share/glib-2.0/schemas/org.gnome.desktop.background.gschema.xml
- /usr/share/glib-2.0/schemas/org.mate.background.gschema.xml
The allowed values are:
- none
- wallpaper
- centered
- scaled
- stretched
- zoom
- spanned
Changing from "zoomed" to "zoom".